One special feature offered in mobile communications systems is group communication. Conventionally group communication has been available in trunked mobile communications systems, such as Professional Radio or Private Mobile Radio (PMR) systems, such as TETRA (Terrestrial Trunked Radio), which are special radio systems primarily intended for professional and governmental users, such as the police, military forces, oil plants.
Group communication with a push-to-talk feature is one of the essential features of any PMR network. Generally, in group voice communication with a “push-to-talk, release-to-listen” feature, a group call is based on the use of a pressel (push-to-talk button) as a switch. By pressing the pressel the user indicates his/her desire to speak, and the user equipment sends a service request to the network. The network either rejects the request or allocates the requested resources on the basis of predetermined criteria, such as the availability of resources, priority of the requesting user, etc. At the same time, a connection is also established to other users in the specific subscriber group. After the voice connection has been established, the requesting user can talk and the other users can listen on the channel. When the user releases the pressel, the user equipment signals a release message to the network, and the resources are released. Thus, the resources are reserved only for the actual speech transaction or speech item, instead of reserving the resources for a “call”.
The group communication is now becoming available also in public mobile communications systems. New packet-based group voice and data services are being developed for cellular networks, especially in the evolution of the GSM/GPRS/UMTS network. In some approaches, the group communication service, and also one-to-one communication, is provided as a packet-based user or application level service so that the underlying communications system only provides the basic connections (i.e. IP connections) between the group communications applications in the user terminals and the group communication service. The group communication service can be provided by a group communication server system while the group client applications reside in the user equipment or terminals. When this approach is employed for push-to-talk communication, the concept is also referred to as Push-to-talk over Cellular (PoC) network. Push-to-talk over Cellular is an overlay speech service in a mobile cellular network where a connection between two or more parties is established (typically) for a long period, but the actual radio channels in the air interface are activated only when somebody is talking. This corresponds to the use of the traditional radio telephones where the radio frequency used is agreed between the parties, and whenever somebody wants to talk he/she presses the push-button, which activates the radio transmission on the selected channel. Alternatively, a voice activity detector (VAD) or any suitable means can be used instead of the manual switch.
The PoC system may also be provided with multiple session support, meaning that a user may be a member of several communication groups. In that case, a user may participate in two or more simultaneous group “calls”, also referred to as “sessions”. However, the user is able to receive speech from one group at a time. If there are two or more incoming streams at the same time, only one of them can be transmitted to the user. It is possible to set one of the communication groups as a priority communication group. The priority communication group may be selected by the network. The priority communication group may be the group in which the current session was first initiated, or it may the group in which the current speech was first transmitted. Alternatively, the priority communication group may be predetermined by the user; for example, it may be a group of particular relevance to the user. If there are two or more incoming voice streams at the same time, only the stream of the higher priority is transmitted to the user.
A problem associated with the above arrangement relates to a situation where the user has just spoken to a “non-priority” communication group and is waiting for a response from that non-priority communication group. If at the same time someone starts to speak from the priority communication group, the dialog from the priority communication group is transmitted to the user, and thus the user is not able to hear the eventual response from the non-priority group, although the response from the non-priority group might be of particular relevance to the user at that moment.